The present invention relates to a surface treated steel sheet for electronic components, used for electronic components of electric products, excellent in solderability, a rust-proof property and a whisker-proof property, and not containing substances with environmental impact such as lead, etc.
For electronic components of electric products, a tinplate, a terneplate, a solder plated steel sheet or the like, as a surface treated steel sheet particularly excellent in solderability, is generally used.
As an example, a surface treated steel sheet having an Sn plated layer of 8.4 to 11.2 g/m2 on each surface of a steel sheet (hereunder referred to as xe2x80x9c#75 to #100 tinplatexe2x80x9d) has been used as a surface treated steel sheet having such excellent solder wettability as to be soldered by dipping the steel sheet in a molten solder bath for a short period of time. However, a space between electronic components has narrowed due to the recent downsizing of electric products, and as a result, tinplate has produced the problems of direct short circuits, the destruction of an insulating layer, and the like, caused by acicular single crystals (whiskers) grown from a tin plated layer. For that reason, a terneplate or a solder plated steel sheet which does not generate whiskers has mainly been used.
As methods for preventing the generation of whiskers, advocated have so far been the method of applying alloy plating (Japanese Examined Patent Publication No. S58-2598, Japanese Unexamined Patent Publication No. S49-129, etc.) and the method of applying post-treatment after plating (Japanese Examined Patent Publication Nos. S56-47955 and S56-47956, Japanese Unexamined Patent Publication Nos. S59-143089 and S62-77481, etc.). However, those methods have scarcely been put to practical use because the alloy plating or the post-treatment hinders the solderability. In the meantime, there is a method which is put to practical use by the optimization of an alloy composition and a chromate treated layer as disclosed in Japanese Unexamined Patent Publication Nos. H2-270970 and H3-183796.
In recent years, the regulations against hazardous substances with environmental impact have been enforced in view of global environmental problems and, in particular, hexavalent chromium and lead are objects of regulation. Therefore, in addition to the need for a material with which lead-tin solder is substituted, the need for a material as a plated steel sheet with which a terneplate or a solder plated steel sheet is substituted is getting increased. In the case of small-sized electronic components in particular, they are soldered by dipping in a molten solder bath for a short period of time so that they are soldered with high efficiency.
Further, as the electronic components are sometimes stored for a long period of time, the performance of the electronic components after long storage is evaluated in the accelerated manner by applying an accelerating treatment in a retort or the like for experimentally reproducing the long storage conditions, and a flux is also being substituted with a type of material having a low activity and not containing chlorine. For a surface treated steel sheet for electronic components, remarkably excellent solder wettability including solder wettability after the accelerating treatment is required.
As described above, there is a strong demand to provide a surface treated steel sheet with less environmental impact for electronic components excellent in both solder wettability and a whisker-proof property.
The object of the present invention is to provide a surface treated steel sheet for electronic components which does not include lead, which is a hazardous substance with environmental impact, and, in particular, satisfies the solder wettability after retort treatment, a rust-proof property and a whisker-proof property, simultaneously.
The present invention is a surface treated steel sheet capable of securing good a whisker-proof property and a good rust-proof property which have been problems in tinplate, while securing more excellent solder wettability, after retort treatment, than a terneplate used for application to electronic components which are currently soldered by dipping in a molten solder bath for a short period of time.
Such a surface treated steel sheet can be attained by; in a surface treated steel sheet for electronic components having an Snxe2x80x94Zn alloy layer which is formed by plating Sn and Zn on a steel sheet or a Ni plated steel sheet and then applying thermal diffusion treatment, or by plating Snxe2x80x94Zn alloy on a steel sheet or a Ni plated steel sheet: specifying the amount of Snxe2x80x94Zn alloy and a Zn/Sn ratio; and applying thereon an inorganic film mainly composed of magnesium phosphate as the substitution of a conventional chromate film.
That is, the present invention is a surface treated steel sheet with less environmental impact for electronic components: excellent in solder wettability, a rust-proof property and a whisker-proof property; and having an Snxe2x80x94Zn alloy layer which is formed by plating Sn and Zn on a steel sheet or a Ni plated steel sheet and then applying thermal diffusion treatment, or by plating Snxe2x80x94Zn alloy, characterized in that: the amount of said Snxe2x80x94Zn alloy layer is not less than 3 g/m2; the Zn/Sn ratio (in weight ratio) is 0.01 to 10, more preferably 0.01 to 0.1; and an inorganic film mainly composed of magnesium phosphate is formed in the amount of 1 to 100 mg/m2 in terms of the amount of P+Mg on said Snxe2x80x94Zn alloy layer.